Monitoring Arctic thin ice: A comparison between Cryosat-2 SAR altimetry data and MODIS thermal-infrared imagery

Abstract. Areas of thin sea ice in the polar regions are not only experiencing the highest rate of sea-ice production but are, therefore, also important hot spots for ocean ventilation as well as heat and moisture exchange between the ocean and the atmosphere. Through co-location of (1) Moderate Resolution Imaging Spectroradiometer (MODIS) derived thin-ice thickness estimates with (2) an unsupervised waveform classification (UWC) approach and (3) Sentinel-1 A/B SAR reference data, thin-ice based waveform shapes are identified, referenced, and discussed with regard to a manifold of waveform shape parameters. Here, a strong linear dependency is found that shows the possibility to either develop simple correction terms for altimeter ranges over thin ice or to directly adjust current retracker algorithms specifically to very thin sea ice. This highlights the potential of CryoSat-2-based SAR altimetry to reliably discriminate between thick sea ice, open-water leads, as well as thin-ice occurrences within recently refrozen leads or mere areas of thin sea ice. Furthermore, a comparison to the ESA Climate Change Initiative's (CCI) surface-type classification reveals that the newly found thin-ice related waveforms are divided up between almost equally between 'unknown' (46.3 %) and lead-type (53.4 %) classifications. Overall, the UWC results in far fewer 'unknown' classifications (1.4 % to 38.7 %). Thus, UWC provides more usable information for sea-ice freeboard and thickness retrieval while UWC at the same time reduces range biases from thin-ice waveforms processed as regular sea ice in the CCI classification.